The need to store digital files, documents, pictures, images and other data continues to increase rapidly. In connection with the electronic storage of data, systems incorporating more than one storage device have been devised. In general, using a number of storage devices in a coordinated fashion in order to store data can increase the total storage volume of the system. In addition, data can be distributed across the multiple storage devices such that data will not be irretrievably lost if one of the storage devices (or in some cases more than one storage device) fails. An additional advantage that can be achieved by coordinating the operation of a number of individual storage devices is improved data access and/or storage times. Examples of systems that can provide such advantages can be found in the various RAID (redundant array of independent disks) levels that have been developed.
For instance, RAID level 0 implements a striped disk array in which data is broken into blocks or strips that are written to separate disk drives. This has the effect of improving the data access and storage times as compared to, for example, a system in which all of the data blocks are stored on a single disk drive, by spreading the individual input/output (IO) requests across a number of physical devices. Furthermore, RAID level 0 can be implemented with two or more disk drives. However, RAID level 0 is not fault tolerant. That is, a failure of even one storage device within a RAID level 0 array results in the irretrievable loss of data from that array.
RAID level 1 is an example of a fault tolerant RAID scheme. According to RAID level 1, data stored in a primary drive is mirrored on a secondary drive. Accordingly, RAID level 1 requires at least two drives to implement. Furthermore, if more than two drives are desired, additional drives are added in pairs. That is, RAID level 1 requires an even number of disk drives. During normal operation, read operations are made with respect to the primary copy of data on the primary drive, and write operations result in a primary copy of data being written to the primary drive and a mirrored copy being written to the secondary drive. If one disk drive within a RAID level 1 array fails, data stored on that drive can be rebuilt onto a replacement disk drive by copying the data stored on the failed drive's companion drive to the replacement drive. However, RAID level 1 does not provide improved IO performance as compared to storing data on a single disk drive. Furthermore, because the write transaction rate during normal operation is doubled, certain implementations of RAID level 1 can result in decreased performance as compared to storing data on a single disk drive.
Another example of a storage array is RAID level 0+1. In general, RAID level 0+1 comprises paired RAID level 0 arrays. As a result, data can be striped across multiple drives, improving IO performance. By striping copies of the data across additional disk drives, redundancy is provided. The disadvantages of RAID level 0+1 include the minimum requirement of four disk drives and the need to maintain an even number of drives in arrays incorporating more than four disk drives.
In RAID level 1+0 two or more drives are mirrored together, and then the mirrors are striped together. Like RAID levels 1 and 0+1, a RAID level 1+0 configuration provides fault tolerance if one drive from a disk set is lost. However, the IO performance of a RAID 1+0 array is not as good as a striped array without mirroring, such as RAID level 0.
Other RAID levels combine independent data disks with parity, either stored on a dedicated parity disk or distributed among data disks. Examples of such arrangements include RAID levels 3, 4, 5 and 6. Although such arrangements provide for fault tolerance, and can provide somewhat improved IO performance, they all require at least three drives to implement, and require fairly complex controller and parity generation circuitry or software.